1. Field of the Invention
The present invention is generally related to a method and apparatus for the early detection of a gas kick in a well bore and, more particularly, to a method which includes comparing a measured value for the mud density in a riser segment at a point just above the blowout preventer (BOP) with either a measured value of the average mud density in the entire riser or a predicted value of mud density determined from a mud flow dynamics model wherein a gas kick is detected by an unfavorable comparison of the two values.
2. Description of the Prior Art
A riser in an offshore drilling operation is a large section of pipe that extends from a blowout preventer to the sea surface. A drill string runs down through the riser and through the BOP to a rotary drill bit connected at its lower end. The bit may be powered by a surface motor or a down hole motor. Drilling fluid, such as mud, is pumped down the drill string through the drill bit to flush cuttings from the bit as well as to cool the bit. The drilling fluid then flows back up to the surface in the annular space defined first by the well itself, and then by the inside of the riser and the outside of the drill string. Below the blowout preventer, the well may be lined with casings to maintain stability of the formations through which the well is drilled. The bell nipple is the top section of the riser where the drilling fluids are removed. The drilling fluids flow into a tank at the surface where the drill cuttings are separated out and, from time to time, various additives are mixed with the drilling fluids to maintain desired properties therein. A pump then re-circulates the drilling fluids back down the drill string for continued use of the fluids.
During drilling, formation gas may enter the well bore to create a "gas kick". If the gas kick is not detected and controlled it may result in a blowout condition of the well. Previous methods of detecting a gas kick have included monitoring the differential flow of mud during a drilling operation and measuring the circulation pressure. In differential flow detection, a substantial increase in the rate of return mud flow without a corresponding increase in the input flow is indicative of an impending blowout. One drawback with differential flow detection is that long integrating periods are required to observe small differential flow and during this time a large quantity of gas kept compressed to a small volume by the hydrostatic head of the mud above it may move up the well and enter the riser before remedial action can be taken. In circulation pressure detection, the pressure required to circulate the drilling fluid through the well is monitored and represents the sum total of all pressure drops through out the system. Fluctuations in the circulation pressure indicate when substantial changes in well bore conditions have occurred; however, they do not indicate when subtle changes in well bore conditions have occurred. Both differential flow detection and circulation pressure detection are performed near the surface at a point quite remote from the point of compressed gas influx. Moreover, rig heave for the semisubmersible or ship-shape floating rig can complicate surface measurements as can the volume changes due to the additions of additives at the surface and/or removal of excess mud due to additive additions.
U.S. Pat. No. 3,595,075 to Dower discloses a method and apparatus for sensing down hole conditions in a well bore. The circulating pressure and the differential pressure are measured at the surface where the mud is pumped down the drill string. The measured differential pressure is used to compute a drill pipe pressure which should be equivalent to the measured circulating pressure. Differences between the measured and computed drill pipe pressure are indicated by a pair of concentric gauges and are attributable to changes in down hole conditions. The apparatus cannot determine what caused the change and would be insensitive to small changes since 80% of the total pressure drop occurs at the drill bit nozzles.
U.S. Pat. No. 3,760,891 to Gadbois discloses a blowout and lost circulation detector which utilizes trend analysis of the returned rate of flow of the drilling fluid. A pressure sensor and fluid column arrangement is used for flow rate measurement. Detection is accomplished by comparing a current measurement with previous measurements and differences that occur are indicative of changes in the system.
U.S. Pat. No. 3,955,411 to Lawson discloses a method for measuring the average density of drilling fluid columns in marine risers. The hydrostatic pressure of the drilling fluid is measured at a point just above the blowout preventer. It is known from the laws of physics that a column of fluid (liquid or gas) exerts a pressure in all directions which is a function of the density of the fluid and the height of the column. Since the height of the column is known, the density of the fluid in the column can be directly derived from the pressure measurement. This pressure measurement must be corrected for a velocity dependent pressure drop. A column of formation gas entering the well bore and rising to a point above the pressure sensor will result in a reduction in the average density of the fluid column. Measuring the average density of the drilling fluid in a whole riser is not a sensitive method for detecting the onset of a gas kick. In deep water, a substantial amount of gas must enter the riser above the detection point before a noticeable reduction in the average density of the fluid in the riser occurs. By this time, a substantial amount of gas would need to be cleared from the well and riser before normal drilling could be resumed.
U.S. Pat. No. 4,408,486 to Rochon et al discloses a bell nipple densitometer for continuously determining the amount of entrained gas present in drilling mud before the gas is released to the atmosphere. The bell nipple has been modified to accommodate two vertically spaced differential pressure measurement ports. Changes in the differential pressure are proportional to changes in the weight of the drilling mud caused by the presence of entrained gases. The bell nipple densitometer disclosed by Rochon et al is not intended to be used as a gas kick detector and cannot be adapted for use as a gas kick detector since the measurement is made at the top of the riser. Conceivably, the riser could be completely full of gas before any change is detected and at this point a blowout could not be stopped. In addition, the vertically spaced measurement ports are only 8.35 inches apart so that the differential pressure measurement can be directly translated into pounds per gallon of water. This spacing can provide accurate measurements after the gas has expanded as it has by the time it reaches the surface, but it would be inapplicable for measurements made on formation gas 6,000 feet below sea level.
U.S. Pat. No. 4,527,425 to Stockton discloses a system for detecting blowout or lost circulation conditions in a borehole. The system uses a "Doppler effect" to detect the change in flow rate between the mud being pumped down through the bit and the mud circulating up the riser. The measurement technique takes advantage of the fact that during mud flow, phase shifts in sonic signals are proportional to the direction and the rate of mud flow. The advantage of this system is that the measurements are made at the bottom of the borehole where the source of the problem is encountered. However, the Stockton device is designed to detect high pressure fluid influx rather than gas influx. Gas aeration may greatly attenuate the acoustic communication between the transmitter and receiver and; therefore, this method is impractical for detecting gas kicks. Moreover, gas influx is the more dangerous cause of blowouts.
U.S. Pat. No. 4,620,189 to Farque discloses parameter telemetering from the bottom of a deep borehole. A frequency modulated signal of a subsurface parameter such as pressure is telemetered to the surface via the conductors of a power cable by superposition of the telemetry data onto the power signals. At the surface the signal is demodulated to produce a signal indicative of the transmitted parameter.